1. Field of the Invention
This invention relates to a crystal pulling apparatus for growing rod-like semiconductor single crystals in a crucible receiving melt and a crystal pulling method utilizing the crystal pulling apparatus to grow the semiconductor single crystal whose impurity concentration is altered in a stepwise fashion.
2. Description of the Related Art
Conventionally, when a rod-like semiconductor single crystal (which is also called ingot) is grown from the melt in the crucible by the Czochralski technique (CZ technique), impurity concentration distribution C in the longitudinal direction of the grown single crystal is given as follows as is well known in the art. EQU C=kC.sub.0 (1-G).sup.k-1
where k is a segregation coefficient of dopant, C.sub.0 is an initial impurity concentration and G is solidification rate. Thus, the impurity concentration distribution in the longitudinal direction of he single crystal is almost definitely determined by the type and initial amount of dopant.
For the above reason, in the CZ technique, it is necessary to pull and grow single crystal ingots of the same number as or more than the required number of impurity concentration levels when it is required to form a certain number of single crystal wafers having impurity concentration levels which are significantly different from one another. Therefore, the efficiency of producing single crystals is lowered. Further, when the necessary amount of single crystal wafers having the desired impurity concentration is relatively small with respect of the amount of a single ingot, a large portion of the produced ingot is wasted, thus lowering the yield of material. When the range of desired impurity concentration distribution is extremely narrow and a large number of wafers having the desired impurity concentration is required, it becomes necessary to grow a large number of ingots in order to obtain the desired number of wafers. Also, in this case, a large portion of the ingots are wasted, lowering the yield of material.
A floating zone technique (FZ technique) is also known to grow single crystals. According to this method, it is possible to grow a crystal having desired impurity concentration values in desired portions thereof in the longitudinal direction of one single crystal ingot. In this case, the above requirements can be satisfied without lowering the efficiency of single crystal production and the yield of material. However, it is generally known that the distribution of dopant impurity in the cross section of the single crystal formed by the FZ technique is extremely non-uniform in comparison with that of the single crystal formed by the CZ technique. When, for example, (100) Si single crystal is used, in-plane distribution .DELTA..rho. of resistivity .rho. of phosphorus (P) doped product is 4 to 10% in the case of CZ technique. In contrast, it is 20 to 50 % in the case of FZ technique ((111) crystal is used in FZ technique). When the maximum and minimum values of .rho. are expressed by .rho..sub.max and .rho..sub.min, then the following equation is obtained. EQU .DELTA..rho.=(.rho..sub.max -.rho..sub.min)/.rho..sub.min